Makoto Osawa

Partitioning behavior of platinum group metals on the γ and γ' phases of Ni-base superalloys at high temperatures

Abstract The partitioning behavior of platinum group metals (PGMs) on the γ and γ′ phases of Ni-base superalloys has been investigated with alloys containing 5 mass% Re and 1 at.% PGMs. It was shown that Ru partitions preferably into the γ, while Rh, Pd, and Pt partition into the γ′, and Ir partitions almost equally. The partitioning behavior is not affected by the 1 at.% PGM addition.

Rafting mechanism for Ni-base superalloy under external stress: elastic or elastic–plastic phenomena?

Abstract Rafting mechanism in Ni-base single-crystal superalloys has been discussed with the total mechanical energy calculated for typical microstructures. We found that the actual rafting phenomena cannot be explained within the coherent elastic regime. The present calculations reveal that (i) only the transverse rafted structure with laminates normal to the stress direction can be realized, regardless of tensile or compressive stresses, and (ii) the lattice misfit is not relevant to the choice of the rafted structures. However, when the eigenstrain of the spherical (dilatational) symmetry changes into that of the tetragonal symmetry with misfit dislocations on the γ/γ′ interfaces, the signs of lattice misfit and external stress govern the choice of the transverse or longitudinal rafts. It is concluded that the rafting belongs to an elastic–plastic phenomenon.

Elastic anisotropy of rafted Ni-base superalloy at high temperatures

Abstract The elastic constants of the rafted Ni-base single-crystal superalloy, TMS-26, at high temperatures (~1000 °C) have been studied with acoustic resonance methods. The rafted superalloy can be regarded as an elastic body of tetragonal symmetry. The elastic anisotropy factors, c 11 / c 33 and E 1 0 0 /E 0 0 1 , increase with temperature up to 1.010–1.025. Although a micromechanics model can reproduce this trend, the calculated anisotropy (~1.001) is much smaller than the measurements, which can be related with the internal strain due to the lattice misfit. The elastic anisotropy of E 0 0 1 1 0 0 =E 0 1 0 contributes to the transverse rafting under external stress.

Elastic constant measurement of Ni-base superalloy with the RUS and mode selective EMAR methods

This paper reports the elastic constants of the Ni-base single crystal superalloy (TMS-26) with a rafted (lamellar) structure having tetragonal symmetry. The elastic constants have been measured at room temperature with the resonance ultrasound spectroscopy method and the mode-selective electromagnetic acoustic resonance method. The value of the elastic constant C33 (250.4 GPa) is almost equal to that of c11 (252.5 GPa), which indicates that the rafted structure virtually has the elastic anisotropy of cubic system.

Interface Energy Calculation of γ-γ′ in Ni-Al System Using the Cluster Variation Method

A calculation of the interface energy for the Ni-Al binary alloy, including the inter-phase boundary (IPB) energy and the anti-phase boundary (APB) energy, has been performed using the Cluster Variation Method (CVM) with the tetrahedron approximation within the temperature range of 600°C~1300°C. The calculated IPB energies range between 8 and 13 mJ/m2, while the APB energies range between 24 and 46 mJ/m2. Additionally, the dependence of the average composition and the order parameter on distance with the compositionally diffuse interfacial regions has been computed. The calculation also shows the width of the diffuse IPB increases with the temperature linearly.

Creep Property and Phase Stability of Sulfur-Doped Ni-Base Single-Crystal Superalloys and Effectiveness of CaO Desulfurization

The direct and complete recycling method for Ni-base superalloy is being developed and studied to reduce the material cost for cost-effective operation of gas turbine systems. Understanding the effect of sulfur contamination is important to determine allowable sulfur content after the recycling. However, in the case of single-crystal superalloys, this effect on material properties is not well known except for the detrimental effect on the oxidation resistance. In the present study, creep tests, aging tests, and cyclic oxidation tests have been performed on PWA1484 with varying sulfur content. The increasing sulfur content has been found to correlate with degradation of properties evaluated here. It is observed that the decrease in creep life in PWA1484 due to sulfur doping is primarily due to coarsening of the γ/γ′ interfacial dislocation network, increase in precipitation kinetics of topologically closed-packed phase, and decrease in oxidation resistance. For recycling purposes, a CaO crucible was used in the casting process, which successfully decreased the sulfur level in the alloy, and the resulting material showed comparable or even better properties in comparison to the low sulfur content material.